This invention relates to a method of manufacturing calcium carbonate single crystal (calcite) widely used as an optical polarizer, etc.
Single crystals of calcium carbonate (CaCO.sub.3) are suitable for optical uses and, for this purpose, natural calcite crystals are currently used. Calcite single crystals exhibit double refraction of incident light. Calcite has a high refractive index and is used as a polarizing prism in optical apparatus. Because of recent advances in the design of optical apparatus, such as laser optics and an optical communication apparatus, there is an increased demand for a material with excellent optical characteristics. In this regard, calcite single crystal is an ideal material, and is expected to be more and more in demand.
Calcite single crystal has been obtained only from natural sources because it is not yet being industrially synthesized. For commercial use, natural calcite must be colorless and transparent, must have no bubbles or cracks and no twining and no internal strains, and must be greater than a certain size. However, calcite single crystal that will meet these requirements is found only in limited quantities in the Republic of South Africa, Mexico, etc., and reserves are running low.
There have been experiments to synthesize calcium carbonate single crystal. One method is crystallization from a solvent, another is the synthesizing from a gel, a third is crystallization from a flux or melt, a fourth is hydrothermal synthesis, and recently an FZ method under high pressure have been suggested. However, optical characteristics such as transparency of the resultant crystals have not been entirely satisfactory due to defects such as impurities, mixing, dislocations, inclusions, or internal strains.
Among the methods tried for the manufacture of calcium carbonate single crystal, hydrothermal synthesis is most similar to the process by which natural calcite is grown as a hydrothermal ore deposit. Therefore, hydrothermal synthesis can produce a desired calcium carbonate single crystal with characteristics similar to natural calcite.
The hydrothermal synthesis process for manufacturing calcium carbonate single crystal utilizes an aqueous solvent hold at a predetermined temperature and pressure in an autoclave. Alkaline aqueous solutions such as sodium hydroxide (NaOH) or alkali carbonate aqueous solutions such as sodium carbonate (Na.sub.2 CO.sub.3), potassium carbonate (K.sub.2 CO.sub.3), etc. are generally used as the aqueous solvent. This method for growth of calcium carbonate single crystal is essentially a modification of conventional growth technology for artificial crystals. Under the following conditions:
Solvent-6 mol K.sub.2 CO.sub.3 aqueous solution
Temperature-410.degree. to 445.degree. C.
Pressure-1720 atmospheres
Growth speed-50 .mu.m/day
about 3 mm growth layer of a calcium carbonate single crystal has been obtained.
The above described hydrothermal synthesis is disclosed in D. R. Kinlock H, R. F. Belt, R. C. Puttbac H, Journal of Crystal Growth 24/25 (1974) 610-613.
A method of manufacturing calcium carbonate single crystal grown by hydrothermal synthesis using a chloride aqueous solution is described and claimed in the commonly assigned U.S. Pat. No. 4,685,995, of Shinichi Hirano and Seiko Instruments & Electronics Ltd., issued Aug. 11, 1987.
In crystal growth method of calcium carbonate utilizing the conventional alkali carbonate aqueous solution, crystal can be grown, but there are problems to be solved as follows:
Firstly, due to the high concentration of solvent, inclusions frequently occur in the resultant crystals. These inclusions will result in inferior optical characteristics. Next, due to a high concentration of solvent, it is impossible to achieve sufficient pressure for quantitative production. In other words, the higher the solvent concentration, the lower the obtained pressure becomes even with the same filling-up rate. In the case of a 6 mol concentration of K.sub.2 CO.sub.3 aqueous solution at 445.degree. C. and a filling-up rate of nearly 100%, it is impossible to obtain a pressure of 1720 atmospheres. Due to this, it is necessary to apply additional pressure from outside the autoclave thereby causing the apparatus and pressure system etc. to be too complicated. Using an alkali carbonate aqueous solution, the growth speed will be very slow, i.e., 50 .mu.m/day, and therefore it will take more or less a year to grow crystals large enough to be used as optical elements.
It is an object of the present invention to provide a simple method to grow relatively quickly excellent calcium carbonate single crystal with good optical characteristics.
According to the present invention, there is provided a method for manufacturing a calcium carbonate single crystal by hydrothermal synthesis at a given temperature and pressure within a nitrate aqueous solution as a solvent.